Expander for electrical seismographs



EXPANDER FOR ELECTRICAL SEISMOGRAPHS Fila-d Feb. 5 3.9 :1

Company, incorporated. New York, N. Y., a

meats,

NT O E y, Dallas,- 'lfein, i

W W-Vacuum Oil it ttionof New York Application rem-um 5, 1941, Serial No. 377,461

1 Claim. (Cl. 177-352) invention relates generally to geophysical I pctina' and more particularly to a method and apparatus for recording seismic waves.

It has long been Mom to those skilled in the art that a detail profile can be made of the interfaces of the subsurface strata from data obtallied by detecting, amplifying and recording i i. c waves. These detected seismic waves can he either reflected or refracted. The instant invention, however, is directed to the detecting, v t lifying and recording of reflected waves. Ex-

perience has taught that the frequency of re-- deeted waves ranges between 40 and 60 cycles and for the greater part are very close to 50.

Wclm. Therefore, for best results it is desirable to have apparatus which will discriminate between different frequencies of detectedjwaves.

in the art of seismic prospecting the. customary promdure is to detonate a charge of explosives at or near the surface of-the earth and at a point removed therefrom detect, amplify and record the waves reflected from the interfaces of subsurface strata. Since a complex train of waves is created by the detonation of the charge of explosives that radiates in all directions from the shot, it is necessary that waves of undesir-' able frequencies be eliminated and those which are reflected from the interfaces of the substrate which carry the most energy be emphasized.

Therefore. the obvious manner for discriminatin 3o hetween these waves is to modify the characteristics of the amplifier used to amplify the detected waves before recording them.

Therefore, the primary object of this invention resides in the provision of means for modifying the characteristics of a seismic amplifier in such a manner that waves of a selected frequency range will. produce an expansion in the gain in amplification as derived from the amplifier and at the same time lessen the effect of waves of uni 40 desirable frequencies. 7

another object of this invention resides in the provision of an amplifier for amplifying detected seismic waves that utilizes the principleof inthe gain in amplification imparted to waves of selected frequencies.

Still another object of this invention resides in the provision of a method of wording reflected w c waves of frequencies in the neighborhood of 50 cycles, in such a manner that reflected warm from the interfaces of. substrate will be recordw at substantially the same amplitude.

a further object of invention resides in in amplification as derived from a seismograph' amplifier in such a manner that the gain in amplification imparted to waves of selected frequency will be dependent upon the energy in the sev lected frequency waves.

Other objects and advantages of this invention will become apparent from the following detailed description. when considered with the accompanyins drawing in which: I

Eigure 1 is a circuit diagram of a seismograph amplifier utilizing the instant invention for controlling the aain in amplification as derived therefrom:

Flam-ems a curve whichhasbeenplottedwith gain in amplification as ordinates and frequency as ,abscissae, showing the manner in which the gain, in amplification derived from the amplifier under a particular condition varies with frequen y; 1 Figure 3 ice curve plotted with the same coorasi igurezshowinsthemannerinwhich gain in amplification varies with an adjustment which is the extreme opposite of that used to produce the curve shown in Figure 2;

quency imder still another condition; ,and

' Figure 5 is still another curve plotted to the same coordinates as Figuresiz, 3 and 4 which 11- lustrates stillanother adjustment of'the elements of the amplifier circuit to, produce; adesired variation in gain with frequency.

Beferrinstothe drawing in detail, particular y Figure l, there'is showli'a detail/circuit diagram 4 of a seismograph amplifierutilizins the rinciplesof the instant invention. In this diagram;

signals generated by a geophone, not shown. are introducedat the terminals l0 and H. These si is are impressed on the grid i! of the thermioni'cinrbe l8. Tube i3 is a multiple element tube having a cathode it, control grid 12, auxiliary grid IS. a screen it a plate lTa. Plate I potential for this tube is supplied from the terverse feed-back for producing an expansion in 4 2i.v thenthrough the top portion of resistance 23 and conductors fl and 2l. The control grid potential which serves as a bias for tube is is the drop produced across the resistance 26. signals passing through the amplifying tune I! are conducted through the condenser 21 to the grids the of a method of controlling the gain 2B and a of the dual triodev type tube a. Grids Y Figure 4 is a curve plotted to the same coordias Figures 2 and 3 showing variationv in gain as derived from the amplifier; with fre ii and iii of tube it are connmted in parallel. The plate circuits and cathode circuits of tube time separate. Plate ii of tube til delivers signals through the condenser iii to grid it of a second dual triode type tube M. Plate iii of tube iii delivers mgnals through the condenser it to grid d1 of tube it. Thecathodes oi tube it are connected in parallel and to ground at it through a conventional resistance and condenser M and it respectively. Both tubes 30 and it receive their plate potential from the same source oi supply marked B+. The potential is supplied to tube lit through the conductor i1 and resistances ti and 42 while the plate potential for tube it is supplied from B+ through the resistance it. Signals passing through the three tubes are amplified and taken on at the point it and recorded by means of a conventional recording galvanometer, not shown. Tube M, receiving signals on its respective grids from the separate plate circuits of time it. acts as an assimilating means whereby the signals are brought together for recording.

The cathodes it and it of tube it are connected to separate circuits. Both circuits are provided with the conventional self-biasing elements comprising the condenser ill and resist ance it for one and the condenser it and the resistance it for the other. The two cathode circuits are connected together by means of a decoupling resistance ii. The circuit of cathode it oi the tube it also includes a condenser iii and resistances iii and lit. Resistances t3 and it have shunted across them in variable relationship to them condensers tit and t thereby forming a capacitative impedance in the circuit of this cathode. In the circuits of cathode it of tube it there is also disposed a condenser-iii and resistances it and ti all connected in series. Superimposed across the resistances W and id but connected in variable relationship with the two resistances is an inductance it. The center points between resistances ti and M and resistances it and it are connected through the conductors 6i and M respectively and conductor it to ground. There is also connected in the circuit of cathode iii oi tube at between the resistance lit and ground, a resistance it having a variable tap as thereon by means of which voltages of a selective order can be impressed upon the grid it of tube ill. Tube t'iis also of the dual triode type. There is connected in the circuit of cathode M of tube it between resistance is and ground, a resistance til that is also provided with a variable tap it by means of. which signal voltages can be taken 0d and impressed on the grid 10 of tube 61.

The plate potential for tube t'i is supplied through the conductor i1, choice it, conductor is and resistance Ii to plate ll, of tube iii. The path of plate potential supplied to plate iii of tube ti is the same as that described in connection with plate it up to the point it. From this oint it is fed through the resistance is to plate 13. Plate potential is also supplied in this manner through resistance it and the conductors it and Ti respectively to plate contacts it and iii of tube it, which is a dual diode rectifying tube. The outputs from the plates of tube iii are impressed on the plates ti and d2 of tube lit. In passing through tube it, the signals are rectified to a direct current voltage which is conducted through the conductors it and it to an auxiliary grid it in tube it where it will produce an exascents pension in the gain in amplification as derived from that tube. v

The operation of this circuit is based on degeneration in the cathode circuits of the dual triocle tube it. If resistances are introduced in the cathode circuits then voltages developed by the tube will appear across the cathode resistances. This voltage in the cathode circuits is out of phase with the voltage developed in the tube so that-degeneration takes place and the mu of the tube till is reduced. The plate load resistors are made small compared to the cathode resistances so that a greater part of the developed voltages appear in the cathode circuits. Since one of the cathode circuits of tube til includes the inductonce it shunted across resistances 5t and t9, the impedance is low at low frequency. therefore there is little or no degeneration of signals at low frequencies.

In the other cathode circuit of tube 30' the condensers it and it are shunted across the resistances ti and ti. If the condensers are selected of proper value. the impedance for higher frequencies will be reduced so that control of the passage of higher frequencies can be obtained. This efiect is best illustrated with reference to Figures 2, 3, i and 5. In Figure 2 there is shown a curve which has been plotted with gain in amplification as ordinates and frequency as abscissae. This curve resulted from introducing all of the inductance 6d into the cathode circuit of cathode it of tube iii and cutting out all of the capacity of the same tube. It will be noted that for low frequencies, maximum gain of amplification is obtained and as the frequency is increased, the gain in amplification drops off sharply. The curve'oi Figure 3, plotted with the same coordinates, illustrates the same efi'ect produced with all of the inductance cut out and all of the capacity cut in. It will be noted that this curve is substantially the opposite of that illustrated in Figure 2, that is, the gain for low frequency is maximum, increasing with increasing frequency. The curve in Figure 4, plotted with the same coordinates, illustrates the eflect produced by cutting out all of the inductance as well as all of the capacity. This curve simulates the characteristics one would expect from a conventional amplifier. Figure 5, still another curve plotted to the same coordinates, is the opposite to the curve of Figure 4, that is. all of the inductance was introduced into one of the cathode circuits and all of the capacity introduced into the other cathode circuit.

From these four curves which are representative of extreme adjustments of the capacity and inductance in the cathode resistance of tube til, it is apparent that any in-between values by proper adjustment can be obtained. These curves serve well to represent the expansion control voltage obtainable by means of cathode degeneration.

In seismic prospecting the predominant frequency for reflected waves in the particular area is ascertained and adjustments of the inductance tit and capacity it made relative to their respective resistances. These elements then serve as a filter network which will pass with maximum emciency the selected frequency band which is that of the reflected waves. These voltage signals passing on through the dual triode tube t! are amplified and rectified by tube and used as a positive bias potential for tube It to produce an expansion of the gain in amplification as derived from this tube.

Thus it can be seen that the advent of inverse feedback makes it possible to construct an expander that will operate on a selected band of frequencies and that the operation of the circuit is based upon the fact that variations of the bias potential on the tube 53 will result in gain variations.

The variable taps on resistances 53 and 54 as well as on resistances 58 and 55 should be ganged for simultaneous operation so that either the inductance or the capacity can be varied at will, separately or simultaneously. Part of the voltages developed across the cathodes in the circuits of cathode M5 or M of tube 30 is applied directly to the tube l3 and part through tubes 6'! and 80 to the auxiliary grid of tube I3. It will be noted that tubes 67 and Bil have a push-pull type connection so that they will operate in a push-pull manner for cancelling any distortion developed.

In operation signals of selected frequencies are divided at the tube 30 and a portion of the signal voltages travel directly on through tube 34 where they are amplified and recorded while the other portion of the signals impressed on tube ill passes through the cathode circuits of this tube where degeneration of undesired frequencies occurs and selected frequencies amplified, rectified and used to control the gain in amplification as derived from the tube l3. In this manner an expander is produced which will operate to expand the gain imparted to the waves of selected frequencies and at the same time produce deterioration of the undesired frequencies.

I claim:

In a seismograph circuit having means for detecting seismic waves by generating electrical signals corresponding to the seismic waves, said signals being of a plurality of definite frequencies, thermionic tube amplifying means for amplifyingthe electrical signals of all of the frequencies, and means for recording the electrical signals in coordination with time, the improvement which comprises means for diverting a portion of the signals passing through the amplifier, a filter for suppressing the signals of undesirable frequencies from the diverted portion 01' the signals, means for rectifying the signal voltages of the selected frequency, and means for applying the rectified voltages to the control grid of at least one of the thermionic amplifier tubes to control the gain in amplification imparted to all signals passing through that tube independent of their frequencies. 7

- LM D. McCARTY. 

